The Janus kinases (JAKs) are cytoplasmic protein tyrosine kinases that play pivotal roles in pathways that modulate cellular functions in the lympho-hematopoietic system that are critical for cell proliferation and cell survival. JAKs are involved in the initiation of cytokine-triggered signaling events by activating through tyrosine phosphorylation the signal transducers and activators of transcription (STAT) proteins. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as transplant rejection and autoimmune diseases, as well as in solid and hematologic malignancies such as leukemias and lymphomas and in myeloproliferative disorders, and has thus emerged as an interesting target for drug intervention.
Four members of the JAK family have been identified so far: JAK1, JAK2, JAK3 and Tyk2. Unlike JAK1, JAK2 and Tyk2, whose expression is ubiquitous, JAK3 is mainly found in hematopoietic cells, JAK3 is associated in a non-covalent manner with the γc subunit of the receptors of IL-2, IL-4, IL-7, IL-9, IL-13 and IL-15. These cytokines play an important role in the proliferation and differentiation of T lymphocytes. JAK3-deficient mouse T cells do not respond to IL-2. This cytokine is fundamental in the regulation of T lymphocytes. In this regard, it is known that antibodies directed against the IL-2 receptor are able to prevent transplant rejection. In patients with X severe combined immunodeficiency (X-SCID), very low levels of JAK3 expression as well as genetic defects in the γc subunit of the receptor have been identified, which indicates that immunosuppression is a consequence of an alteration in the JAK3 signaling pathway.
Animal studies have suggested that JAK3 not only plays a critical role in T and B lymphocyte maturation, but also that JAK3 is required to maintain lymphocyte function. Modulation of the immunological activity through this new mechanism can prove useful in the treatment of T cell proliferative disorders such as transplant rejection and autoimmune diseases.
JAK3 has also been shown to play an important role in mast cells, because antigen-induced degranulation and mediator release have been found to be substantially reduced in mast cells from JAK3 deficient mice. JAK3 deficiency does not affect mast cell proliferation nor IgE receptor expression levels. On the other hand, JAK3−/− and JAK3+/+ mast cells contain the same intracellular mediators. Therefore, JAK3 appears to be essential in the IgE-induced release of mediators in mast cells and its inhibition would be, thus, an effective treatment for allergic reactions.
In conclusion, JAK3 kinase inhibitors have been recognised as a new class of effective immunosuppressive agents useful for transplant rejection prevention and in the treatment of immune, autoimmune, inflammatory and proliferative diseases such as psoriasis, psoriatic arthritis, rheumatoid arthritis, multiple sclerosis, inflammatory bowel diseases, systemic lupus erythematosus, type I diabetes and complications from diabetes, allergic reactions and leukemia (see e.g. O'Shea J. J. et al, Nat. Rev. Drug. Discov. 2004, 3(7):555-64; Cetkovic-Cvrlje M. et al, Curr. Pharm. Des. 2004, 10(15):1767-84; Cetkovic-Cvrlje M. et al, Arch. Immunol. Ther. Exp. (Warsz), 2004, 52(2):69-82).
Accordingly, it would be desirable to provide novel compounds that are capable of inhibiting JAK/STAT signaling pathways, and in particular which are capable of inhibiting JAK3 activity, and which are good drug candidates. Compounds should exhibit good activity in in vitro and in vivo pharmacological assays, good oral absorption when administered by the oral route, as well as be metabolically stable and exhibit a favourable pharmacokinetic profile. Moreover, compounds should not be toxic and exhibit few side effects.